3.2. Lists

A list is an ordered collection of items, and is indicated in Python via square brackets ([]), all individual elements being separated by commas. There need not be any particular relation between the elements, and each element can be of a different data type as well. Some examples are given below:

>>> a = ['hello',2,float(5),9,7,'wow',8]
>>> a
['hello', 2, 5.0, 9, 7, 'wow', 8]

The elements within a list are indexed starting from 0 and can be accessed using the square bracket ([]) operator, as shown below. Python also allows us to compute the index of an entry by passing the value to the index operator. Unlike arrays in Java or C++, negative indices are valid and point to elements from the end of the list, i.e., an index of -1 points to the last element, an index of -2 points to the second last element, and so on. A list can also be sliced through using the operator [start:end], which includes all elements with indices between start and end-1. If start is not specified, it defaults to 0, and if end is not specified, it defaults to the end of the list. For example:

>>> print a[1]
2
>>> print a[-1]
8
>>> print a.index(9)
3
>>> a[1:4]
[2, 5.0, 9]
>>> a[1:-1]
[2, 5.0, 9, 7, 'wow']
>>> a[:5]
['hello', 2, 5.0, 9, 7]
>>> a[2:]
[5.0, 9, 7, 'wow', 8]
>>> a[:]
['hello', 2, 5.0, 9, 7, 'wow', 8]

Individual elements can be modified in a straightforward manner by using the assignment operator (=). Elements can be added to the end of the list using the append operator, and anywhere within the list using the insert operator. Likewise, elements can be deleted from the end of the list by using the pop operator, or anywhere within the list by using the del keyword. Multiple elements can be added at the same time from the end of the list using the extend operator.

>>> a = ['hello',2,float(5)]
>>> a[0] = 4
>>> a
[4, 2, 5.0]
>>> a.append('wow')
>>> a
[4, 2, 5.0, 'wow']
>>> a.pop()
'wow'
>>> a
[4, 2, 5.0]
>>> a.insert(2,'hello')
>>> a
[4, 2, 'hello', 5.0]
>>> del a[1]
>>> a
[4, 'hello', 5.0]
>>> a.extend([9,7,'wow'])
>>> a
[4, 'hello', 5.0, 9, 7, 'wow']

Warning

Note that if the append operator is used in place of the extend operator above, then it will append a new list to the original list. This is a common error that should be avoided at all costs. The flexibility of a list to simultaneously contain elements of different data types can also be a hazard at times, as no error will be reported back. For example:

>>> a = [4, 'hello', 5.0]
>>> a.append([9,7,'wow'])
>>> a
[4, 'hello', 5.0, [9, 7, 'wow']]

Unlike the append operator, the pop operator returns back the element deleted from the list. If an index is supplied to the pop operator, then it can also be used for deleting an element from anywhere inside the list. Elements can also be deleted by value using the remove operator. For example:

>>> mystring = "Old McDonald had a farm, Duda!"
>>> words = mystring.split()
>>> words
['Old', 'McDonald', 'had', 'a', 'farm,', 'Duda!']
>>> words.pop(0)
'Old'
>>> words
['McDonald', 'had', 'a', 'farm,', 'Duda!']
>>> words.remove('Duda!')
>>> words
['McDonald', 'had', 'a', 'farm,']

Note how we used the split operator to generate a list of all the words in a string. A list can also be sorted using the sort operator. Elements can be arranged in either ascending or descending order, depending on whether the reverse argument is set. A list can also be easily reversed using the reverse operator.

>>> a = [5,2,7,4,9]
>>> a.sort()
>>> a
[2, 4, 5, 7, 9]
>>> a.sort(reverse=True)
>>> a
[9, 7, 5, 4, 2]
>>> mystring = "Old McDonald had a farm, Duda!"
>>> words = mystring.split()
>>> words.sort()
>>> words
['Duda!', 'McDonald', 'Old', 'a', 'farm,', 'had']
>>> words = mystring.split()
>>> words.reverse()
>>> words
['Duda!', 'farm,', 'a', 'had', 'McDonald', 'Old']

Similar to Java or C++, Python assigns an ASCII code to all alphanumeric characters, where all capitalized letters have a lower ASCII code compared to their non-capitalized counterparts. This is why, for example, 'Old' appears before 'a' in the sorted list above. The sort operator sorts all the elements of a list in-place, so the order of the elements does change. One can also use the sorted operator that acts on a list and returns another list which is the sorted version of the original list. Elements can also be sorted in reverse if the reverse argument is set.

>>> a = [5,2,7,4,9]
>>> b = sorted(a)
>>> b
[2, 4, 5, 7, 9]
>>> b = sorted(a,reverse=True)
>>> b
[9, 7, 5, 4, 2]
>>> a
[5, 2, 7, 4, 9]

The length of a list can be obtained by using the len keyword. The addition operator (+) is overloaded for lists to mean concatenation. Note that multiplying a list by a constant does not scale the individual elements by the given constant, instead, it concatenates the original list to itself that many times. The operators += and *= are likewise defined. For example:

>>> a = [5,2,7,4,9]
>>> len(a)
5
>>> b = [2,11,7]
>>> a+b
[5, 2, 7, 4, 9, 2, 11, 7]
>>> b*3
[2, 11, 7, 2, 11, 7, 2, 11, 7]
>>> a+=b
>>> a
[5, 2, 7, 4, 9, 2, 11, 7]
>>> b*=2
>>> b
[2, 11, 7, 2, 11, 7]